The speed and the size of the ink jet recording system have been increased in recent years, and the ink jet recording system has come to be used in commercial printing (hereinafter, written as “ink jet printing”). Industrial printing machines based on the ink jet recording system that are used in commercial printing (hereinafter, written as “ink jet printing machines”) have been developed (see, for example, Patent Literature 1). Because large numbers of copies are printed in the commercial printing field, the printing speed is important due to the tradeoff between productivity and printing costs. The printing speed is 15 m/min or above, and is often 60 m/min or above. For higher speed printing, rotary pigmented ink jet printing machines have been developed which are capable of a printing speed exceeding 120 m/min.
Because the ink jet printing machines allow for handling of variable information, their use is particularly found in on-demand printing. A preferred manner of commercial printing is to print fixed information on an offset printing machine and to print variable information on an ink jet printing machine.
The printing paper used for ink jet printing machines is coated paper, plain paper such as wood free paper for so-called offset printing, or PPC paper. In the field of commercial printing such as invoices, account statements, leaflets, direct mails and so-called TransPromo (transaction and promotion) that is the combination of these printed matters, there is recently a trend for higher image quality and consequently data usually printed on plain paper are increasingly printed on coated paper. When, however, inks are printed on the conventional plain paper or coated paper for offset printing with an ink jet printing machine at the above printing speed, the inks exhibit poor fixation on the printing paper to cause problems such as uneven drying of the inks, and images being contaminated with the inks. In a worse case, marks are left by the flowing of inks (the inks sitting on the paper run on the surface). Further, the printed images are smudged during the handling of the printed paper.
There are two types of ink jet inks used on ink jet printing machines: dye inks that are solutions of color materials in solvents such as water, and pigmented inks that are dispersions of color materials in solvents such as water. The dye inks outperform the pigmented inks in the sharpness of images, but tend to compare unfavorably in terms of the weather resistance of images. The color materials in the dye inks are fixed by being absorbed into the printing paper, whilst the color materials in the pigmented inks are fixed through adhesion to the surface of the printing paper. For both types of the inks, it is important that water as the solvent be quickly absorbed to allow the color materials to dry. That is, ink fixing properties are of importance. With the dye inks, it is also important that the color materials be prevented from being redissolved in the solvents or moisture in the air. In the case of the pigmented inks, strong adhesion of the color materials to the surface of printing paper is critical. Insufficient ink fixing properties cause the occurrence of phenomena such as contamination of prints with the inks. In addition, insufficient fixation of the dye inks causes the occurrence of problems such as the bleeding of images due to the redissohition of color materials, and also beading, thus decreasing sharpness. In the case of the pigmented inks, insufficient adhesion of the color materials decreases rubbing resistance, resulting in problems such as chalking and smudges.
Ink jet recording paper that has a coating layer containing a porous pigment is a known technique for improving the absorptivity with respect to ink jet inks (see, for example, Patent Literatures 2 and 3). To improve the printability with dye inks and pigmented inks, ink jet recording sheets are known that have an ink receiving layer containing a styrene-acrylic copolymer resin, a guanidine compound as a cationic polymer, and a secondary ammonium salt compound. Other ink jet recording sheets are also known which have an ink receiving layer containing at least one of zinc oxide, zinc sulfate, magnesium chloride and magnesium sulfate, and a guanidine compound. Further, ink jet recording media are known which are coated with a coating color including an aqueous emulsion that is obtained by copolymerizing two or more kinds of monomers including an aromatic vinyl monomer and a conjugated diene monomer in the presence of a water-soluble polymer having an alcoholic hydroxyl group. (See, for example, Patent Literatures 4 to 6.)